Lisa Robin Kelly has died at the age of 43 on Wednesday, according to her agent. The actress, best known for her role on "That '70's Show," had endured a years-long battle against addiction.

"Lisa had voluntarily check herself into a treatment facility early this week where she was battling the addiction problems that have plagued her these past few years," said her agent, Craig Wyckoff, according to CNN.

"I spoke to her on Monday and she was hopeful and confident, looking forward to putting this part of her life behind her," he recalled.

Despite Kelly's optimistic attitude, the actress was found dead in the California rehab facility on Wednesday, Aug. 14, according to TMZ.

"Cause of death has not been issued yet and no death certificate has been issued either," Wyckoff noted, reported CNN.

The actress' death comes as a shock in Hollywood where "The Bachelor" star Gia Allemand is still being mourned after also having died on Wednesday. Kelly portrayed Laurie Forman, the sister of Topher Grace's lead character Eric on the hit show.

However, Kelly only appeared on the first three seasons of the show, and was later replaced by a different actress. The FOX series, which ended in 2006, also starred Mila Kunis and Ashton Kutcher.

In the years following "That '70's Show," Kelly grappled with addiction as well as brushes with the law.

In November of 2011, the star was taken into custody along with her husband Robert Joseph Gilliam over a disturbance at their Charlotte, N.C. home. The subsequent mug shot of Kelly later went viral, shocking fans with her gaunt appearance and so different to how fans had grown used to seeing her.

The arrest along with the mug shot led many people to believe that the actress was on drugs. Kelly later admitted a past addiction to drugs, but maintained that she had gotten clean.

"I had lost a baby, as a result of that, I lost everything," said the actress, according to the Daily Mail. "And I was abusing alcohol, which I no longer do. With the 70's Show, I was guilty- of the drinking problem- and I ran. And I am not running from this. And I have paid my dues."

Furthermore, just two months ago, Kelly was arrested under the suspicion of drunk driving.

Meanwhile, fans are mourning Kelly on Twitter following news of her death on Wednesday, including actress Mackenzie Phillips, among several other celebrities.